The present invention relates to a finely patterned mask for fabrication of semiconductor devices, a process for production of the mask, and a process for fabricating semiconductor devices with the mask.
The ever increasing degree of integration and miniaturization of LSI (large scale integrated circuit) needs a new technology for its realization, such as electron beam projection lithography (EPL), which includes PREVAIL (projection exposure with variable axis immersion lenses) and LEEPL (low energy electron beam proximity projection lithography).
PREVAIL is a so-called reduction projection exposure technique. It is designed to transfer a mask pattern to a resist layer by irradiation with a high- energy electron beam (for example, 100 keV), which has passed through a 4× stencil mask. The mask pattern is reduced to ¼ for transfer by a lens system. The mask that has been proposed for use in PREVAIL is a stencil mask which is a silicon membrane about 2 μm thick, for example, with a pattern or aperture formed therein.
The stencil mask for PREVAIL works in the following manner. The aperture forming the mask pattern permits the electron beam to pass through without scattering, and the electron beam that has passed through the aperture is focused on the resist layer, so that the mask pattern is transferred onto the resist layer. The electron beam incident on that part of the stencil mask, which has no mask pattern, is scattered by silicon atoms, and the scattered beams are screened by the limiter plate. The stencil mask is thick enough (2 μm) to cause this scattering. An excessively thin stencil mask does not function because it permits an electron beam to pass without scattering.
LEEPL is a so-called 1:1 electron beam exposure technique, which employs a 1:1 stencil mask. It is designed to transfer a 1:1 pattern to the resist layer by irradiation with a low-energy electron beam (about 2 keV). The mask that has been proposed for use in LEEPL is a stencil mask which is a silicon membrane (or thin film) or a diamond membrane, both about 500 nm thick, with a pattern or aperture formed therein. The stencil mask for LEEPL works in the following manner. That part of the mask where the aperture is formed permits the electron beam to pass through, so that the mask pattern is transferred onto the resist layer.
FIG. 12 shows a process for making a stencil mask used for conventional electron beam transfer lithography, such as LEEPL. The process starts with making a mask blank 4, which consists of a silicon substrate 1, an etching-resistant layer 2, and a membrane layer 3, as shown in FIG. 12A. The etching-resistant layer 2 functions as an etching stopper when the silicon substrate 1 undergoes selective etching. The etching-resistant layer 2 may be a silicon nitride (SiN) film if the membrane layer 3 is formed from diamond. Alternatively, the etching-resistant layer 2 may be a silicon oxide (SiO2) film if the membrane layer 3 is formed from silicon (Si). In the latter case, the mask blank is a so-called SOI (silicon on insulator) substrate.
Then, the membrane layer 3 is coated with a resist and the resulting resist layer is patterned by exposure and development. Thus, there is obtained the resist mask 5, as shown in FIG. 12B. The membrane layer 3 undergoes selective etching (dry etching) through this resist mask 5. Thus, the aperture or the mask pattern 6 is formed in the membrane layer 3.
Next, the silicon substrate 1 undergoes selective etching on its reverse side, such that its peripheral part remains unetched and the part corresponding to the mask region is removed, as shown in FIG. 12C. During this selective etching, the membrane layer 3 remains intact owing to the etching-resistant layer 2.
Finally, the etching-resistant layer 2 is selectively removed by etching through the remaining part of the silicon substrate 1 as a mask, as shown in FIG. 12D. Thus, there is obtained the desired stencil mask 7.
On the other hand, there has been proposed a stepper mask for an electron beam in Japanese Patent Laid-open No. Hei 11-54409. It is constructed such that the membrane layer is divided into sections by reinforcing joists.
In the meantime, the above-mentioned stencil mask 7 has some problems to be solved. If it is to have finer patterns with higher precision, the membrane layer 3 has to be thinner than before. Unfortunately, the membrane layer 3 with a reduced thickness is so weak that the mask pattern 6 is broken when the stencil mask is cleaned or mounted on the exposure tool. Moreover, the thin membrane layer 3 with a large area distorts to adversely affect the positioning accuracy.
The membrane layer (or thin film) of the stencil mask should preferably be as thin and stiff as possible so that apertures (as fine mask patterns) can be made therein. A membrane layer, several to tens of millimeters square in area, is necessary for projection of an LSI chip pattern through a 1:1 transfer mask by scanning with an electron beam (beam of charged particles). The aperture as the mask pattern is usually formed by dry etching, and the ratio of its size to its depth is limited to about 1:10. For example, the membrane layer should be thinner than 500 nm for an aperture of 50 nm in size. For this reason, the membrane layer is formed from a material with a high Young's modulus, such as diamond, under the condition, which evolves a high tensile stress. A mask pattern (or aperture) formed in such a membrane layer distorts due to tensile stress. In addition, this tensile stress restricts the design of the wiring pattern for LSI. The wiring pattern needs a thin long “crossbar” that crosses each space between wires, and its length is limited by tensile stress.
A mask of the stepper type with a membrane layer divided into sections by reinforcing joists has the disadvantage that there occurs a seam between shots in the pattern. In addition, it is difficult to support the membrane layer with thin joists arranged at equal intervals. As a result, the membrane layer is liable to stress concentration, which not only deforms but also destroys the mask easily.